Interlock circuitry for microwave oven

ABSTRACT

A microwave oven has first and second interlock switches operative when the door is opened to apply power to the filament only of the magnetron and when the door is closed to apply power to the magnetron plate. If either switch malfunctions when the door is opened, the other operative switch interrupts the power to the magnetron plate.

United States Patent [72] lnventor Harry F. Chapell Maynard, Mass. [211 App]. No. 24,538 [22] Filed Apr. 1, 1970 [45] Patented Nov. 30, 1971 [73] Assignee Sage Laboratories, Inc.

Natick, Mass.

[54] INTERLOCK ClRCUlTRY FOR MICROWAVE OVEN 8 Claims, 2 Drawing Figs.

[52] U.S.CI 219/1055 [51] Int. Cl 1105b 9/06 [50] Field of Search 219/1055 [56] References Cited UNITED STATES PATENTS 3,317,699 5/1967 Helfer 219/1055 3,407,279 10/1968 Greenberg 219/1055 3,523,170 8/1970 Boehm 219/1055 Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. .laeger An0rneyWolf, Greenfield and Sacks ABSTRACT: A microwave oven has first and second interlock switches operative when the door is opened to apply power to the filament only of the magnetron and when the door is closed to apply power to the magnetron plate. If either switch malfunctions when the door is opened, the other operative switch interrupts the power to the magnetron plate.

RES INPUT COM TIMER I04 PATENTEI] NUV30 I971 SHEET 2 OF 2 INPUT ATTORNEYS 1 INTERLOCK CIRCUITRY FOR MICROWAVE OVEN BACKGROUND OF THE INVENTION The present invention relates in general to microwave ovens and more particularly concerns interlock circuitry for use with a microwave oven to provide safe operation. The invention helps avoid harmful radiation from the microwave oven source and turns off the empty microwave oven when the oven door is closed with relatively inexpensive interlock circuitry capable of performing a number of interlock actions.

It is an object of the invention to provide cooperating interlocks for interrupting microwave power if either interlock malfunctions.

It is another object of the invention to provide interlock delay circuitry for lessening harmful radiation even when both interlocks fail in an unsafe position.

Another object of the invention is to reduce the chance of an unsafe failure by providing dual interlocks, one of which interrupts the power to the microwave source directly; the other, indirectly, such as by way of a relay.

Still another object of the invention is to provide for automatic turn-on of the magnetron filament when the oven is initially opened.

Still another object of the invention is to provide interlock circuitry that automatically resets the oven timer to zero when the oven door is opened.

Another object of the invention is to provide interlock circuitry which deenergized the microwave energy source after a suitable time delay when no food is in the oven and the oven door is shut.

Still another object of the invention is to provide interlock circuitry which will interrupt the power to the filament and plate circuits of the microwave oven when the door is left open for at least a predetermined time interval.

Still another object of the invention is to achieve one or more of the foregoing objects at low cost with apparatus relatively free from complexity.

SUMMARY OF THE INVENTION According to the invention, a microwave oven includes interiock circuitry with an input terminal coupled to an electric power supply, such as the l-volt AC line. First and second switching means initially carry power from the input terminal to the filament of a microwave energy source, such as a magnetron. Means are also provided for applying power to the plate of the microwave energy source after applying power to the filament thereof. The first and second switching means are also adapted to interrupt power to the plate when the door is opened.

Numerous other features, object and advantages of the invention will now become apparent from the following specification when read in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram showing the microwave interlock circuitry according to the invention; and

FIG. 2 shows an additional safety feature that may be used with the circuitry of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS On-olf switch 12 and circuit breaker l4 carry AC energy to normally closed tube thermostat switch 16 in thermal contact with magnetron 40. If magnetron 40 overheats, switch 16 opens to interrupt power to magnetron 40.

Normally input power is coupled to contact 20C of switch 20 and to terminal 82A of relay contact 82 operated by relay 80. Switches 20 and 22 are shown in FIG. I with the door closed. Assuming that power has just been applied by turning switch 12 on, power flows from contact 20A to terminal 92A of relay contacts 92. Contacts 92 are then open, and magnetron 40 receives no power. Contacts 22A and 22C of switch 22 are then interconnected.

When the door is opened, if, and only if, switches 20 and .22 are properly operating, contacts 20C and 20B of switch 20 and contacts 22B and 22C of switch 22 complete a conducting path to terminal A of relay 80. Relay 80 then latches to close its contacts 82, thereby connecting the input power via terminals 82A and 823 through filament transformer 46 to the magnetron filament. This power is, in turn, applied to the filament 40B of tube 40. Relay 80 also then provides power to heater element 70 of time delay switch 68. Switch 68 delays application of plate power for a minimum interval, typically 10 seconds, after the filament is energized. After this delay interval, contact 69 of switch 68 closes, thereby enabling timer unit to receive power via switch 22 and contact 69.

When the oven door closes, switches 20 and 22 again assume the position shown in FIG. 1. The power across relay 80 is then transferred through contacts 22A and 22C of switch 22 and contact 69 of delay switch 68 to timer unit 100. If the door closes before the delay interval expires, contact 69 is still open, and timing unit 100 receives no power until the delay interval ends.

Timing unit 100 includes a bank of switches 110, a bank of resistors 120, a switch release solenoid 102 and solid state timer 104. The switch bank 110 includes a number of switches 110A through 110F, typically mounted on the front of the microwave oven, and a plurality of switches 122A through 122F operative with respect switches 110A through lIOF to connect respective resistors 120A through 120F across terminals and 106 of solid state timer 104. Switch release solenoid 102 is coupled to all switch contacts to cause them to release, thus assuming the position shown in FIG. 1, when solenoid 102 is energized. Solid state timer 104 is operative when it receives an input on line 104A through one of the selected switches A through 1 10F. The timer 104 includes reset line 104B and output line 104C.

Timer 104 also includes an internal switch (not shown) for coupling power from input line 104A and the common contact connected thereto to output line 104C for a time interval dependent upon the value of a resistor connected between terminals 105 and 106 associated with the internal delay switch (not shown). After this time interval, the internal switch interrupts the fiow of power to output line 104C and applies power to reset line 1043.

In operation, one puts food in the oven, closes the door and selects the cooking time by pressing one of the switches of timer unit 100. An input signal passes through time delay switch 68 and switch bank 110 to line 104C and relay 90. Contact 92 on relay 90 then closes to energize primary winding 43 of plate transformer 42. Upon completion of the selected cooking time, the internal switch of timer I04 switches power to line 1043, thereby energizing solenoid I02 and resetting all switches in timing unit 100 to the position shown in FIG. I. This action also releases relay 90 to deenergize primary winding 43 of plate transformer 42.

Under normal operation, when the cooking time ends, relay 90 unlatches to deenergize magnetron 40. When the oven is then opened and either switch 20 or 22 is properly functioning, the power that is still available at contact 20C of switch 20 is coupled through contact 20B of switch 20 to relay 102 to assure that it has released or reset the timing unit 100.

In microwave ovens it is desirable to keep the filament voltage on if the oven is going to be used quite often. However,

when the oven is not going to be used for say, three minutes the filament power should be turned off thereby providing longer life for the filament itself. The circuitry of FIG. 1 provides for filament power turnoff after a predetermined time when the oven door is closed. Time delay switch 75 provides this feature. When no food is being cooked, for example, and the switch band 110 is in the released position, power is applied to heater element 75. After a predetermined time interval of 180 seconds, for example, contact 76 opens, deenergizing relay 80 and turning off filament power. Relay 80 can again be energized only by opening the oven a second time.

The safety interlock circuitry shown herein prevents harm to an operator of the oven if the oven is opened when the mag netron is radiating energy. A number of interlocks help prevent the escape of potentially harmful radiation under nearly all conditions.

If the door is opened while food is cooking, for example, and either switch or switch 22 malfunctions, power is available through either contact 203 of switch 20 or contact 228 of switch 22 to release solenoid 102. This releases relay 90 to open contact 92, thereby deenergizing the plate 40A of magentron 40.

Switch 20 interrupts the power to magnetron 40 directly, and switch 22 interrupts the power indirectly through releasing relay 90 and opening contact 92. The chance of both interlock switches sticking at the same time is quite remote.

If both switch 20 and 22 stick in an unsafe position, when the door is opened, magnetron 40 will not then shut off. However, an operator should at once see that the magnetron is still operating and close the door. With the door closed the cooking time will eventually end and contacts 92A-92B will open. At that time the oven door can be opened safely. if the door remains closed, is reclosed, or if switch 22 sticks in an unsafe (door normally closed) position when in fact it is open, then power through switch bank 110 energizes heater element 77. After the three minute delay contact 76 opens to deenergize relay 80 thereby interrupting all power to the control circuit and magnetron. Furthermore, relay 80 will not relatch after contact 76 opens and when door is opened if either switch 20 or switch 22 is in its unsafe position. Thus, the malfunctioning interlock switch has to be replaced before the oven can be operated thereby preventing unsafe operation.

Referring to FIG. 2 there is shown an added feature that may be used with the circuitry of FIG. 1. As previously mentioned, when no food is being cooked and the door is left closed relay 80 is eventually deenergized and no power is fed to the filament or plate. If the door is left open, however, the filament could be energized indefinitely thereby significantly reducing its lifetime. The circuitry of FIG. 2 prevents this by limiting the on" time of the filament particularly when the door is left open.

Relay 30 with associated contacts 32 and 34 and a time delay switch 24 having a heater element 26 and a contact 25 are connected so that if an operator of the oven leaves the door open for a predetermined time, all power to the magnetron is interrupted. This interlock is operative, when either switch 20 or switch 22 has malfunctioned, and the oven door is left open.

Normally when the oven door is open and switchs 20 and 22 are operating, they are as shown in FIG. 2. After a predetermined time the heater element 26 of time delay switch 24 heats sufficiently to close the normally open contacts 25 and latch relay 30. Contacts 32 and 34 of relay 30 then open and interrupt all power through contact 82 of relay 80 or switch 22 directly.

When the door is open and either switch 20 or switch 22 sticks in a door closed position, power is available by way of the operative switch, such as switch 20 contacts 208 and 20C, to beat heater element 26. This eventually latches relay 30, opens contacts 32 and 34 and interrupts power to the filament after a predetermined time, typically 180 seconds. When the oven is to be used again, the door has to be closed initially, thereby opening contact 20, deenergizing relay 30 and providing power to switch 22.

There has been described a novel arrangement for safely operating a microwave oven while minimizing the chances of unsafe radiation occurring, and an arrangement for providing extended life for the magnetron filament. It is evident that those skilled in the art may now make numerous uses of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.

What is claimed is: g 1. Safety and interlock circuitry for use with a microwave oven having a door ad a source of microwave energy comprising;

an input terminal for receiving electrical power, means for initially establishing a first circuit condition when said door is opened including first and second switch means,

said first and second switch means being coupled from said input terminal and responsive to door for each assuming a first position when said door is open and a second position when said door is closed,

said first and second switch means being arranged when said door is open to establish the first circuit condition when both said first and second switch means are in their first position,

means for coupling electron heating power from said input terminal to said microwave energy source in response to said first circuit condition,

means for thereafter setting a second circuit condition for a preselected cooling time interval when both said door is closed and said first circuit condition is established,

and means for coupling energy from said input terminal to said microwave energy source to produce microwave radiation therefrom in response to said second condition, whereby the malfunction of at least one of said first and second switch means so that either stays in its second position prevents setting of said first circuit condition thereby indicating a fault condition of said switch means.

2. Safety and interlock circuitry in accordance with claim 1 and comprising a first latching means coupled from said first and second switch means whereby said latching means is actuated when said door is opened and both said first and second switch means properly assume their first position.

3. Safety and interlock circuitry in accordance with claim 2 wherein said first latching means includes a relay and said means for coupling electron heating power includes a conductive connection coupled from one side of said relay to the filament of said source.

4. Safety and interlock circuitry in accordance with claim 1 wherein said means for setting said second circuit condition includes timing means for setting said preselected interval and means responsive to completion of said preselected interval for resetting said timing means.

5. Safety and interlock circuitry in accordance with claim 4 and further comprising first time delay means for establishing a predetermined minimum time interval between occurrence of said first circuit condition and the occurrence of said second circuit condition in response to the occurrence of said first circuit condition.

6. Safety and interlock circuitry in accordance with claim 4 and further comprising second time delay means responsive to the occurrence of said timing means being reset for interrupting power from said input terminal to said microwave energy source that produces microwave radiation therefrom after a predetermined time interval following such resetting.

7. Safety and interlock circuitry in accordance with claim 1 and further comprising third time delay means responsive to said door left closed for interrupting power to said microwave energy source after a preselected time interval.

8. Safety and interlock circuitry for use with a microwave oven having a door and a source of microwave energy comprising;

an input terminal for receiving electrical power,

and second switch means assuming their first position,

and time delay means coupled to at least one of said first and second switch means for interrupting said means for coupling said electron heating power after a predetermined time interval, when said door is open and at least one of said first and second switch means assumes its first position. 

1. Safety and interlock circuitry for use with a microwave oven having a door ad a source of microwave energy comprising; an input terminal for receiving electrical power, means for initially establishing a first circuit condition when said door is opened including first and second switch means, said first and second switch means being coupled from said input terminal and responsive to said door for each assuming a first position when said door is open and a second position when said door is closed, said first and second switch means being arranged when said door is open to establish the first circuit condition when both said first and second switch means are in their first position, means for coupling electron heating power from said input terminal to said microwave energy source in response to said first circuit condition, means for thereafter setting a second circuit condition for a preselected cooking time interval when both said door is closed and said first circuit condition is established, and means for coupling energy from said input terminal to said microwave energy source to produce microwave radiation therefrom in response to said second condition, whereby the malfunction of at least one of said first and second switch means so that either stays in its second position prevents setting of said first circuit condition thereby indicating a fault condition of said switch means.
 2. Safety and interlock circuitry in accordance with claim 1 and comprising a first latching means coupled from said first and second switch means whereby said latching means is actuated when said door is opened and both said first and second switch means properly assume their first position.
 3. Safety and interlock circuitry in accordance with claim 2 wherein said first latching means includes a relay and said means for coupling electron heating power includes a conductive connection coupled from one side of said relay to the filament of said source.
 4. Safety and interlock circuitry in accordance with claim 1 wherein said means for setting said second circuit condition includes timing means for setting said preselected interval and means responsive to completion of said preselected interval for resetting said timing means.
 5. Safety and interlock circuitry in accordance with claim 4 and further comprising first time delay means for establishing a predetermined minimum time interval between occurrencE of said first circuit condition and the occurrence of said second circuit condition in response to the occurrence of said first circuit condition.
 6. Safety and interlock circuitry in accordance with claim 4 and further comprising second time delay means responsive to the occurrence of said timing means being reset for interrupting power from said input terminal to said microwave energy source that produces microwave radiation therefrom after a predetermined time interval following such resetting.
 7. Safety and interlock circuitry in accordance with claim 1 and further comprising third time delay means responsive to said door left closed for interrupting power to said microwave energy source after a preselected time interval.
 8. Safety and interlock circuitry for use with a microwave oven having a door and a source of microwave energy comprising; an input terminal for receiving electrical power, means for establishing a first circuit condition when said door is opened including first and second switch means, said first and second switch means being responsive to said door for each assuming a first position when said door is open and a second position when said door is closed, means for coupling electron heating power from said input terminal via said first and second switch means to said microwave energy source in response to both said first and second switch means assuming their first position, and time delay means coupled to at least one of said first and second switch means for interrupting said means for coupling said electron heating power after a predetermined time interval, when said door is open and at least one of said first and second switch means assumes its first position. 